Circuits and methods for a static random access memory using vertical transistors

ABSTRACT

A memory cell. The memory cell has a flip-flop that includes a cross-coupled pair of inverters. The inverters each include a pair of complementary, vertical transistors. A gate contact interconnects the gates of the inverters and acts as the input of the inverter. A shunt interconnects a first source/drain region of the complementary transistors and acts as the output of the inverter. A first vertical, access transistor is also included. The first vertical, access transistor has a gate that is coupled to a word line, a first source/drain region that is coupled to the output of one of the inverters, and a second source/drain region that is coupled to a first bit line. A second vertical, access transistor is also provided. The second vertical, access transistor has a gate that is coupled to the word line, a first source/drain region that is coupled to the output of the other inverter, and a second source/drain region that is coupled to a second bit line.

This application is a Divisional of U.S. Ser. No. 09/028,727, filed Feb.25, 1998, U.S. Pat. No. 6,304,483.

FIELD OF THE INVENTION

The present invention relates generally to semiconductor integratedcircuits. More particularly, it pertains to circuits and methods for astatic random access memory using vertical transistors.

BACKGROUND OF THE INVENTION

Modem electronic systems typically include a data storage device such asa dynamic random access memory (DRAM), static random access memory(SRAM) or other conventional memory device. The memory device storesdata in vast arrays of memory cells. Each cell conventionally stores asingle bit of data (a logical “1” or a logical “0”) and can beindividually accessed or addressed.

Electronic systems, e.g., computers, conventionally store data duringoperation in the memory device. As these systems become moresophisticated, they require more and more memory in order to keep pacewith the increasing complexity of software based applications that runon the systems. Thus, as the technology relating to memory devices hasevolved, designers have tried to increase the density of memory cells inthe memory device. The electronics industry strives to decrease the sizeof the memory cells. This allows a larger number of memory cells to befabricated without substantially increasing the size of thesemiconductor wafer.

Static random access memory or “SRAM” is one type of memory device thatis used with computers. Conventionally, an SRAM device includes an arrayof addressable memory cells. Each cell includes a four transistorflip-flop and access transistors that are coupled to input/output nodesof the flip-flop. Data is written to the memory cell by applying a highor low logic level to one of the input/output nodes of the flip-flopthrough one of the access transistors. When the logic level is removedfrom the access transistor, the flip-flop retains this logic level atthe input/output node. Data is read out from the flip-flop by turning onthe access transistor.

Memory devices are fabricated using photolithographic techniques thatallow semiconductor and other materials to be manipulated to formintegrated circuits as is known in the art. These photolithographictechniques essentially use light that is focussed through lenses todefine patterns in the materials with microscopic dimensions. Theequipment and techniques that are used to implement thisphotolithography provide a limit for the size of the circuits that canbe formed with the materials. Essentially, at some point, thelithography cannot create a fine enough image with sufficient clarity todecrease the size of the elements of circuit. In other words, there is aminimum dimension that can be achieved through conventionalphotolithography. This minimum dimension is referred to as the “criticaldimension” (CD) or minimum feature size (F) of the photolithographicprocess.

The minimum feature size imposes one constraint on the size ofconventional SRAM memory cells. Conventionally, SRAM cells have used asurface area on a substrate that is approximately equal to 120 featuresquares. Recently, researchers have designed an SRAM cell in an area ofapproximately 100 feature squares. In order to keep up with the demandsfor higher capacity memory devices, designers need to further reduce thesize of the memory cells.

For the reasons stated above, and for other reasons stated below whichwill become apparent to those skilled in the art upon reading andunderstanding the present specification, there is a need in the art foran SRAM cell which uses less surface area than conventional SRAM cells.

SUMMARY OF THE INVENTION

The above mentioned problems with memory devices and other problems areaddressed by the present invention and will be understood by reading andstudying the following specification. A circuit and method for a staticrandom access memory cell are described which use vertical transistorswith a surface area of approximately 32 feature squares (“F²”) toimplement a six transistor cell.

In particular, an illustrative embodiment of the present inventionincludes a memory cell. The memory cells has a flip-flop that includes across-coupled pair of inverters. The inverters each include a pair ofcomplementary, vertical transistors. A gate contact interconnects thegates of the inverters and acts as the input of the inverter. A shuntinterconnects a first source/drain region of the complementarytransistors and acts as the output of the inverter. A first vertical,access transistor is also included. The first vertical, accesstransistor has a gate that is coupled to a word line, a firstsource/drain region that is coupled to the output of one of theinverters, and a second source/drain region that is coupled to a firstbit line. A second vertical, access transistor is also provided. Thesecond vertical, access transistor has a gate that is coupled to theword line, a first source/drain region that is coupled to the output ofthe other inverter, and a second source/drain region that is coupled toa second bit line.

In another embodiment, a method for forming a memory cell is provided.The method includes forming vertical bars of semiconductor material on asurface of a semiconductor wafer. The bars have vertically aligned firstsource/drain, body and second source/drain regions. Individual pillarsof semiconductor material are separated out from the vertical bars toform vertical transistors. Vertical transistors from adjacent bars withbody regions of different conductivity types are coupled to forminverters. The inverters are cross-coupled to form an array offlip-flops. Vertical access transistors are coupled to inputs/outputs ofthe flip-flops.

In another embodiment, an electronic system is provided. The electronicsystem includes a microprocessor. The microprocessor is coupled to astatic random access memory. The memory has an array of six-transistormemory cells. Each memory cell is formed with six vertical transistorson a surface area of approximately 32 feature (F) squares.

In another embodiment, a memory device is provided. The memory deviceincludes a number of word lines and a number bit lines that are disposedto form an array. A number of memory cells are addressably disposed atintersections of word and bit lines. Each memory cell includes across-coupled pair of inverters that are formed from verticaltransistors. Each memory cell also includes a pair of vertical, accesstransistors that are each coupled to the output of one of the inverters.Each vertical, access transistor in a memory cell is coupled to aselected word line and a selected bit line. A word line decoder iscoupled to the word lines of the memory array and selectively activatesthe word lines of the array. A sense amplifier is coupled to the bitlines of the array of memory cells. A bit line decoder is coupled to thesense amplifier and selects bit lines for reading and writing data toand from the memory cells.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view illustrating generally an embodiment of aportion of a static random access memory array according to theteachings of the present invention.

FIG. 2 is a top view of the embodiment of FIG. 1.

FIG. 3 is a schematic diagram of the embodiment of FIG. 1.

FIGS. 4A through 4P illustrate an embodiment of a process forfabricating an array for a static random access memory device accordingto the teachings of the present invention.

FIG. 5 is a block diagram of an embodiment of a static random accessmemory device constructed according to the teachings of the presentinvention.

DETAILED DESCRIPTION

In the following detailed description of the invention, reference ismade to the accompanying drawings which form a part hereof, and in whichis shown, by way of illustration, specific embodiments in which theinvention may be practiced. These embodiments are described insufficient detail to enable those skilled in the art to practice theinvention. Other embodiments may be utilized and structural, logical,and electrical changes may be made without departing from the scope ofthe present invention.

The terms wafer and substrate used in the following description includeany structure having an exposed surface with which to form theintegrated circuit (IC) structure of the invention. The term substrateis understood to include semiconductor wafers. The term substrate isalso used to refer to semiconductor structures during processing, andmay include other layers that have been fabricated thereupon. Both waferand substrate include doped and undoped semiconductors, epitaxialsemiconductor layers supported by a base semiconductor or insulator, aswell as other semiconductor structures well known to one skilled in theart. The term conductor is understood to include semiconductors, and theterm insulator is defined to include any material that is lesselectrically conductive than the materials referred to as conductors.The following detailed description is, therefore, not to be taken in alimiting sense.

The term “horizontal” as used in this application is defined as a planeparallel to the conventional plane or surface of a wafer or substrate,regardless of the orientation of the wafer or substrate. The term“vertical” refers to a direction perpendicular to the horizontal asdefined above. Prepositions, such as “on”, “side” (as in “sidewall”),“higher”, “lower”, “over” and “under” are defined with respect to theconventional plane or surface being on the top surface of the wafer orsubstrate, regardless of the orientation of the wafer or substrate.

FIG. 1 is a perspective view illustrating generally an embodiment of aportion of a static random access memory array according to theteachings of the present invention. Specifically, memory cell 100 is asix transistor memory cell that is formed using, for example, thetechnique described below with respect to FIGS. 4A through 4P. Eachtransistor in memory cell 100 is formed in a vertical pillar of singlecrystalline semiconductor material, e.g., silicon. In this embodiment,memory cell 100 is formed on a semiconductor-on-insulator (SOI)structure that includes substrate 113 and insulator layer 112.

Memory cell 100 includes transistors Q₁ and Q₂ which act as accesstransistors for memory cell 100. Transistors Q₁ and Q₂ are n-channelmetal-oxide-semiconductor (MOS) transistors each with vertically alignedn+ source/drain regions and a p− body region. Transistors Q₁ and Q₂ areformed in a pillar of semiconductor material that extends outwardly fromn+ rails 102 and 103, respectively. Throughout this specification thedesignation “n+” refers to semiconductor material that is heavily dopedn type semiconductor material, e.g., monocrystalline silicon orpolycrystalline silicon. Similarly, the designation “p+” refers tosemiconductor material that is heavily doped p type semiconductormaterial. The designations “n−” and “p−” refer to lightly doped n and ptype semiconductor materials, respectively. Rails 102 and 103 act as thebit lines (B/L₁ and B/L₂, respectively) for memory cell 100 and areformed integrally with one of the n+ source drain regions of the accesstransistors. The gates of transistors Q₁ and Q₂ are coupled to wordline,W/L.

Memory cell 100 also includes a flip-flop formed of cross-coupledinverters 104 and 105. Inverter 104 comprises the combination ofvertical transistors Q₃ and Q₄. The gates of transistors Q₃ and Q₄ arecoupled together by gate contact 106. Gate contact 106 is disposed alonga side of transistors Q₃ and Q₄. Transistor Q₃ is a n-channel transistorand transistor Q₄ is a p-channel transistor. Transistors Q₃ and Q₄ arecoupled to similar transistors in other cells by n+ rail 107 and p+ rail108, respectively. Rail 107 is coupled to ground potential and rail 108is maintained at a higher, voltage supply level. Transistors Q₂, Q₃, andQ₄ are coupled together with shunt 109. Similarly, transistors Q₅ and Q₆are coupled together to form a second inverter with gate contact 110 andshunt 111. Shunt 111 also couples the inverter to access transistor Q₁.The inverters are cross coupled by coupling shunt 109 with gate contact110 and coupling shunt 111 with gate contact 106.

The n-channel and p-channel transistors of memory cell 100 have gatesthat are formed of n+ and p+ polysilicon, respectively. The polysilicongates in an inverter are coupled together with a gate contact that isformed of a refractory metal so as to provide a dual work functionfeature for desired surface channel characteristics in each transistorin the inverter. It is noted that the device bodies of the transistorsin memory cell 100 are isolated from each other and the substrate suchthat the transistors exhibit semiconductor-on-insulator characteristics.Thus, the transistors may be fully depleted, floating body devices andno CMOS wells are needed for isolation. However, a body contact can beincluded using the technique of U.S. application Ser. No. 08/889,396,entitled Memory Cell with Vertical Transistor and Buried Word and BodyLines (the '396 Application). The '396 Application is incorporated byreference.

The operation of the embodiment of FIG. 1 is described in connectionwith the schematic diagram of FIG. 3. Data is written to and read frommemory cell 100. For example, a low logic level is written to memorycell 100 by activating access transistor Q₁ with a high logic level onwordline W/L. Bit line B/L₁ is brought to a low logic level. TransistorQ₁ transmits this low logic level to inverter 105 at input/output node A(shunt 111). This low voltage is also passed to gate contact 106 ofinverter 104. This turns off transistor Q₃ and drives input/output nodeB (shunt 109) to a high potential. Node B is coupled to gate contact 110of inverter 105 so as to maintain the low logic level on the node A.Wordline W/L is then reduced to ground potential and memory cell 100stores the low logic level in inverter 105 at node A. In a similarmanner a high logic level is written to memory cell 100 by raising thebit line B/L₁ to a high logic level.

Data is read out of memory cell 100 by activating access transistors Q₁and Q₂ so as to pass the voltage levels of nodes A and B to bit linesB/L₁ and B/L₂, respectively. The voltage on the bit lines is sensed todetermine the stored logic level.

FIG. 2 is a top view of the embodiment of FIG. 1 that illustrates thatmemory cell 100 can be fabricated in a surface area of approximately 32feature squares.

FIGS. 4A-4P illustrate generally an embodiment of a process for formingvertical transistors for a memory cell of a static random access memory(SRAM) device. This embodiment produces a truesemiconductor-on-insulator (SOI) device. Alternatively, a bulktechnology embodiment can be constructed using a p− substrate with asubstrate bias of V_(dd). The process sequence described with respect tothis embodiment assumes a minimum lithographic dimension (CD) of 0.3micrometers (μm). The lateral film thickness may be scaled for othervalues of CD. Further, it is noted that the suggested verticaldimensions are approximate and dependent on required voltage levels andtool tolerances.

As shown in FIG. 4A, the process begins with semiconductor wafer 245.Semiconductor wafer 245 comprises, for example, a monocrystallinesilicon wafer. Semiconductor wafer 245 advantageously comprises a highlydoped substrate to reduce electrical noise in the memory device. Oxidelayer 400 is formed on semiconductor substrate 245 using, for example,chemical vapor deposition (CVD) of silicon oxide (SiO₂). The overallthickness of oxide layer 400 is approximately 0.9 micrometers (μm). Aphotoresist is applied and selectively exposed to provide a parallelstripe pattern at minimum width and spacing. The exposed photoresist isused as a mask to directionally etch through oxide layer 400 to formtrenches 410. The photoresist is removed.

Trenches 410 are filled with a nitride material by, for example, CVD ofsilicon nitride (Si₃N₄). A working surface of the nitride material isplanarized, such as by chemical mechanical polishing/planarization(CMP).

Over the next sequence of steps, trenches 410 are filled with layers ofsemiconductor material that will act as source/drain and body regions ofvertical devices in the SRAM array. Initially, bars of semiconductormaterial are formed in trenches 410. In later steps, portions of eachbar are removed so as to define columns of individual verticaltransistors. Each transistor in the column is the same conductivitytype, e.g., all of the transistors in one column are n-channel devicesor all are p-channel devices. An SRAM cell includes transistors that areformed on four of these bars as shown in FIG. 1. Thus, an SRAM cell isformed using transistors formed from three bars of n-channel verticaltransistors and one bar of p-channel transistors. For sake of clarity inthe drawings, however, the process of forming the p-channel andn-channel transistors is described using only one bar for eachconductivity type. The other two bars are formed in a similar manner asthe pictured n-channel device.

Photoresist layer 420 is applied and selectively exposed to provide amask to cover nitride filled trenches 410 that will house n-channeltransistors. The nitride material is removed from the unmasked trenches.Photoresist layer 420 is removed, such as by conventional photoresiststripping techniques.

FIG. 4B illustrates the structure of the portion of the array after thenext sequence of process steps. Source/drain layer 151 is formed by, forexample, expitaxial growth of n+ silicon in open trenches 410. Theoverall thickness of layer 151 is approximately 0.2 micrometers (μm).Body region layer 156 is formed by epitaxial growth of, for example, p−silicon with a thickness of approximately 0.3 μm on layer 151. Secondsource/drain layer 161 of n+ silicon is formed by, for example,epitaxial growth on layer 155 with a thickness of approximately 0.2 μm.In this manner, bar 432 of semiconductor material is formed in opentrenches 410. A top surface of layer 161 is recessed below a top surfaceof layer 400 by approximately 0.2 μm. Oxide cap 401 is grown on layer161 with a thickness of approximately 20 to 50 nanometers.

The remaining nitride material is removed from trenches 410 to allowformation of p-channel transistors. Source/drain layer 150 is formed by,for example, expitaxial growth of p+ silicon in open trenches 410. Theoverall thickness of layer 150 is approximately 0.2 micrometers (μm).Body region layer 156 is formed by, for example, epitaxial growth of n−silicon with a thickness of approximately 0.3 μm on layer 150. Secondsource/drain layer 160 of, for example, p+ silicon is formed byepitaxial growth on layer 155 with a thickness of approximately 0.2 μm.In this manner, bar 431 of semiconductor material is formed in opentrenches 410. A top surface of layer 160 is recessed below a top surfaceof layer 400 by approximately 0.2 μm. The structure is now as shown inFIG. 4B.

FIG. 4C illustrates the structure after the next series of processsteps. Oxide cap 401 is stripped from bar 432. Nitride layer 405 isformed by, for example, chemical vapor deposition of Si₃N₄ so as to fillthe recesses over bars 431 and 432. Nitride layer 405 is planarizedusing, for example, chemical/mechanical polishing to create a workingsurface of nitride layer 405 that is substantially co-planar with aworking surface of oxide layer 400. A layer of photoresist material isdeposited and exposed so as to expose the array portion of the staticrandom access memory device. Thus, the ends of bars 432 and 432 are leftcovered by the photoresist mask. With the mask in place, oxide layer 400is removed.

Nitride material is deposited using, for example, chemical vapordeposition to form a layer of material with a thickness of approximately20 nanometers. This covers sidewalls 407 of bars 431 and 432. Thenitride material is directionally etched by, for example, a reactive ionetching technique so as to remove the nitride material from horizontalsurfaces. The remaining nitride material forms nitride spacers 408 onsidewalls 407 of bars 431 and 432. It is noted that nitride layer 405 ontop of bars 431 and 432 remains in tact during these steps.

Next, insulator layer 411 is formed between bars 431 and 432 andsubstrate 245 so as to form a semiconductor on insulator (SOI)structure. Oxide layer 411 is formed using, for example, the techniquesof U.S. application Ser. No. 08/745,708, entitled Silicon-On-InsulatorIslands and Method for Their Formation (the '708 Application), or U.S.Pat. No. 5,691,230, entitled Technique for Producing Small Islands ofSilicon on Insulator (the '230 Patent). The '708 Application and the'230 Patent are incorporated by reference. Nitride layer 405 and nitridespacers 408 are removed using, for example, a phosphoric acid etch.

FIG. 4D is an elevational view illustrating the structure after the nextsequence of steps. Oxide layer 445 is formed by, for example, chemicalvapor deposition of silicon oxide to cover bars 431 and 432. Nitride cap460 is formed by, for example, chemical vapor deposition of a nitridematerial with a thickness of approximately 0.1 μm. A photoresist isapplied and selectively exposed to provide a mask which defines aminimum dimension stripe pattern orthogonal to bars 431 and 432. Nitridecap 460 and the oxide layer 445 are etched through, such as by RIE, tothe point where the top surface of bars 431 and 432 are exposed. Thephotoresist is removed by conventional photoresist stripping techniques.

The portions of bars 431 and 432 exposed through layer 460 areselectively etched to expose portions of layers 150 and 151,respectively. The exposed portions of layer 150 and 151 are respectivelyidentified as rails 430 and 440. A top view of this structure is shownin FIG. 4E and a perspective view is shown in FIG. 4F.

FIG. 4G illustrates the structure following the next set of fabricationsteps. A layer of oxide material is thermally grown on the exposedsemiconductor surfaces of bars 431 and 432. This oxide material formsgate insulator 310 on vertical sidewalls 407 as well as oxide layer 312on rails 430 and 440.

FIG. 4H illustrates a cross-sectional view of FIG. 4G, along cut line4H—4H. Gate material 250 is deposited on oxide layer 312 by chemicalvapor deposition of n+ polysilicon. Gate material 250 fills cavities inbars 431 and 432. The gate material is planarized to a level that isapproximately co-planar with a working surface of nitride layer 460.

FIG. 4I illustrates the structure after the next sequence of processsteps. A photoresist is applied and selectively exposed to provide amask which reveals the portions of gate material 250 which arepositioned over the top of the rails 430 (the p+ rails). Gate material250 is etched out, stopping on the oxide layer 312 and gate oxide 310.Gate oxide 310 is removed and then the photoresist layer is alsoremoved. Gate oxide 310 b is formed on the exposed silicon of bar 431.Gate material 251 is deposited by chemical vapor deposition. Gatematerial 251 comprises, for example, p+ polysilicon. Gate material 251is planarized using, for example, chemical/mechanical planarizationtechniques such that a working surface of gate material 251 is co-planarwith layer 460 and gate material 250. The p+ polysilicon gate material251 is better suited for PMOS devices. Gate materials 250 and 251 arenext etched to recess them below the surface of the top of theirrespective bars, 432 and 431. This is accomplished using, for example, areactive ion etching process.

FIG. 4J is a top view of the structure. Nitride spacer layer 500 isdeposited, such as by CVD with a thickness of ¼ of the criticaldimension or minimum feature size (“CD” or “F”) of the process. Nitridespacer layer 500 is etched to leave on the sides of nitride layer 460and oxide layer 445. In one embodiment, nitride spacer layer 500 isetched by reactive ion etching (RIE).

FIG. 4K is a perspective view illustrating the structure after the nextsequence of steps. Photoresist layer 446 is applied and exposed to forma mask which reveals portions of oxide layer 445 that lie betweenadjacent bars 431 and 432 that will house transistors to be coupled toform an inverter. The exposed oxide 445 between gate material 250 and251 is selectively etched through an RIE process using nitride layer 500as a mask. The etching is to be timed to reach a sufficient depth toprovide access for gate contact conduction, but not to a depth whichreaches substrate 245. Next, gate material 250 and 251 respectively, areselectively etched using the overhanging nitride spacer layer 500 as amask. This step leaves n+ and p+ polysilicon gate material on opposingvertical sidewalls 300 a and 300 b. The selective etching is timed to asufficient depth to stop on oxide layer 312. The structure is now as itappears in FIG. 4K.

Next, the nitride spacer layers 500 and nitride cap 460 are stripped.FIG. 4L illustrates the structure after the next series of steps, viewedalong the cut line 4L—4L of FIG. 4K. Gate contact 190 is deposited byCVD to fill the space between opposing gate materials 251. In oneembodiment, gate contact 190 is tungsten. In another embodiment, gatecontact 190 is any other suitable refractory metal. Gate contact 190 isrecessed through an RIE process to approximately the level of the top ofbars 431 and 432, and likewise to a sufficient depth to expose the topsof the gate materials 250 and 251, respectively.

FIG. 4M illustrates the same view after the next sequence of steps. Aphotoresist is applied and exposed to create a mask with stripes thatcover one of the two gate materials 250 and 251 between adjacent pillarsof semiconductor material 252 of bars 431 and 432. The exposed gatematerial 250 and 251 is removed leaving one gate in each trench thatseparates adjacent pillars 252. The photoresist is removed. Intrinsicpolysilicon layer 530 is deposited to fill space left by the removal ofthe gate material 250 and 251. Intrinsic polysilicon layer 530 can beapplied through a CVD process. Intrinsic polysilicon layer 530 is thenplanarized by a CMP process that stops on a working surface of oxidelayer 445.

FIG. 4N illustrates the structure following the next sequence of stepsin the fabrication process. A photoresist is applied and exposed todefine a mask in the openings that extend between adjacent source/drainlayers 160 and 161. Oxide layer 445 is etched to expose the underlyingsecond source/drain regions, 160 and 161. The etching is performed byany number of conventional etching processes, such as RIE. Then, thephotoresist is removed and electrical contact 170 is deposited.Electrical contact 170 is a metal such as tungsten (W) or titanium (Ti).However, in an alternate embodiment, other forms of conductors withsimilar conduction properties may be used. At this point, electricalcontact 170 is planarized until even with intrinsic polysilicon layer530. The structure is now as it appears in FIG. 4N.

FIG. 40 illustrates the structure following the next sequence of stepsin the fabrication process. The electrical contact 170 is recessed byRIE methods to approximately 0.05-0.1 μm. Nitride layer 550 is formedupon the electric contact. Nitride layer 550 is approximately 0.15 μm inthickness and deposited by CVD. CMP is used once more to planarize,leaving nitride layer 550 on top of the electrical contact 170.

FIG. 4P shows the structure following the final sequence of processsteps. A photoresist is applied and exposed to form a mask which is todefine contact studs for gate line contact 190. Intrinsic polysilicon560 is removed. The photoresist layer is also removed. Oxide isdeposited with a thickness of ¼ CD such as by CVD. The oxide isdirectionally etched to leave as spacers 562. Next, contact 180 isdeposited by CVD. In one embodiment, contact 180 comprises a refractorymetal. In an alternative embodiment, the electrical input 180 comprisespolysilicon. A thick dielectric layer is deposited to act as a wiringinsulator. At this point, the process has produced an array of invertersand access transistors that can now be connected through conventionalwiring techniques to form cells of a static random access memory arrayof the type described above with respect to FIGS. 1 through 3.

FIG. 5 is a block diagram of an illustrative embodiment of the presentinvention. This embodiment includes memory device 500 that is coupled toelectronic system 502 by control lines 504, address lines 506 andinput/output (I/O) lines 508. Electronic system 502 comprises, forexample, a microprocessor, a processor based computer, microcontroller,memory controller, a chip set or other appropriate system for readingand writing data in a memory device. Memory device 500 includes array ofmemory cells 510 that is coupled to word line decoder 514 and senseamplifier 511. Array of memory cells 510 comprises a number of staticrandom access memory (SRAM) cells. Each cell includes cross-coupledinverters and two access transistors. The transistors in each cell areformed in vertical pillars as in the embodiment of FIG. 1, above. Array510 is coupled to sense amplifier 511.

Word line decoder 514 includes word line drivers that are coupled toword lines of array 510. Sense amplifier 511 is coupled to bit linedecoder 512. Bit line decoder 512 and word line decoder 514 are coupledto address lines 506. Bit line decoder 512 is coupled to I/O circuit516. I/O circuit 516 is coupled to 1/0 lines 508. Control circuit 518 iscoupled to control lines 504, sense amplifier 511, word line decoder514, bit line decoder 512, and I/O circuit 516.

In operation, electronic system 502 provides signals on address lines506 and control lines 504 when data is to be read from or written to acell of array 510. Word line decoder 514 determines the word line of aselected cell of array 510 using the address provided on address lines506. Further, bit line decoder 512 determines the bit line of theselected cell of array 510. In a read operation, sense amplifier 511detects the value stored in the selected cell based on bit lines ofarray 510. Sense amplifier 511 provides this voltage to I/O circuit 516which, in turn, passes data to electronic system 502 over I/O lines 508.In a write operation, I/O circuit 516 passes data from I/O lines 508 tosense amplifier 512 for storage in the selected cell of array 510.

Conclusion

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat any arrangement which is calculated to achieve the same purpose maybe substituted for the specific embodiment shown. This application isintended to cover any adaptations or variations of the presentinvention. For example, the minimum feature size can be varied from the0.3 μm value disclosed. The teachings of the present invention can beapplied to a six transistor embodiment with depletion mode n-channeltransistors as load devices for the inverters. In another embodiment,the inverters for an array of SRAM cells can be constructed using theteachings of commonly assigned, copending application Ser. No.08/028,805 (attorney Docket No. 303.41OUS1), and filed even dateherewith, entitled Circuits and Methods Using Vertical, ComplementaryTransistors, which application is incorporated herein by reference.

What is claimed is:
 1. A method for forming a memory cell, the methodcomprising: forming vertical bars of semiconductor material on a surfaceof a semiconductor wafer, wherein the bars have vertically aligned firstsource/drain, body and second source/drain regions; separating outindividual pillars of semiconductor material from the vertical bars toform vertical transistors, each transistor being formed from a singlepillar of semiconducting material; coupling vertical transistors fromadjacent bars with body regions of different conductivity types to forminverters, forming each inverter including: interconnecting the gates ofthe vertical transistors with a gate contact, the gate contact acting asan input of the inverter; interconnecting the first source/drain regionsfrom the vertical transistors from adjacent bars with body regions ofdifferent conductivity types with a shunt, the shunt acting as an outputof the inverter; cross-coupling the inverters to form a flip-flop; andcoupling vertical, access transistors to inputs/outputs of theflip-flops.
 2. The method of claim 1, and further comprising forming aninsulator layer beneath the bars of semiconductor material.
 3. Themethod of claim 1, wherein coupling the vertical transistors fromadjacent bars with body regions of different conductivity types to forminverters comprises: forming a tungsten shunt that couples the firstsource/drain regions of the vertical transistors on a top surface of thefirst source/drain regions to provide an output for the inverter.
 4. Themethod of claim 3, wherein cross-coupling the inverters comprisescoupling the gate contacts of each inverter with the shunt of the otherinverter.
 5. A method of forming a memory cell, comprising: forming aflip-flop that includes a cross-coupled pair of inverters whereinforming each inverter includes: forming a pair of complementary,vertical transistors, each transistor being formed from one of a pair ofsingle pillars of semiconducting material; interconnecting the gates ofthe inverter with a gate contact, the gate contact acting as an input ofthe inverter; interconnecting the first source/drain regions of thecomplementary vertical transistors with a shunt, the shunt acting as anoutput of the inverter; coupling a gate of a first vertical accesstransistor to a word line; coupling a first source/drain region of thefirst vertical access transistor to the output of one of the inverters;coupling a second source/drain region of the first vertical accesstransistor to a first bit line; coupling a gate of a second verticalaccess transistor to the word line; coupling a source/drain region ofthe second vertical access transistor to the output of the otherinverter; and coupling a second source/drain region of the secondvertical access transistor to a second bit line.
 6. The method of claim5, wherein coupling a second source/drain region of a vertical accesstransistors to the bit lines includes coupling a second source/drainregion of the vertical access transistors to a layer of dopedsemiconductor material that is integral with the second source/drainregion of the vertical access transistors.
 7. The method of claim 5,wherein coupling a second source/drain region of a vertical accesstransistors to the bit lines includes coupling a second source/drainregion of the vertical access transistors to bit lines formed below thebody regions of the vertical access transistors.
 8. The method of claim5, wherein interconnecting the gates of the inverter with a gate contactincludes interconnecting the gates of the inverter with a tungsten gatecontact that is disposed along a side of the complementary, verticaltransistors of each inverter.
 9. A method of forming a memory cell,comprising: forming a flip-flop that includes a cross-coupled pair ofinverters wherein forming each inverter includes: forming a pair ofcomplementary, vertical transistors, each transistor being formed fromone of a pair of single pillars of semiconducting material;interconnecting the gates of the inverter with a gate contact, the gatecontact acting as an input of the inverter; interconnecting the firstsource/drain regions of the complementary vertical transistors with atungsten shunt, the tungsten shunt acting as an output of the inverter;coupling a gate of a first vertical access transistor to a word line;coupling a first source/drain region of the first vertical accesstransistor to the output of one of the inverters; coupling a secondsource/drain region of the first vertical access transistor to a firstbit line; coupling a gate of a second vertical access transistor to theword line; coupling a source/drain region of the second vertical accesstransistor to the output of the other inverter; and coupling a secondsource/drain region of the second vertical access transistor to a secondbit line.
 10. The method of claim 9, wherein interconnecting the firstsource/drain regions of the complementary vertical transistors with atungsten shunt includes coupling the tungsten shunt to a top surface ofthe first source/drain region of one of the vertical, accesstransistors.
 11. The method of claim 9, wherein forming a memory cellincludes forming a memory cell that uses a surface area of asemiconductor wafer that is approximately 32 feature (F) squares. 12.The method of claim 5, wherein coupling a second source/drain region ofa vertical access transistors to the bit lines includes coupling asecond source/drain region of the vertical access transistors to anintegral bar of semiconductor material that interconnects thetransistors with similar transistors in other cells.
 13. A method offorming a memory cell, comprising: forming a flip-flop that includes across-coupled pair of inverters wherein forming each inverter includes:integrally forming a pair of complementary, vertical transistors on apair of complementary semiconductor rails that run along a surface of asemiconductor-on-insulator wafer, each transistor being formed from oneof a pair of single pillars of semiconducting material; interconnectingthe gates of the inverter with a gate contact, the gate contact actingas an input of the inverter; interconnecting the first source/drainregions of the complementary vertical transistors with a shunt, theshunt acting as an output of the inverter; coupling a gate of a firstvertical access transistor to a word line; coupling a first source/drainregion of the first vertical access transistor to the output of one ofthe inverters; coupling a second source/drain region of the firstvertical access transistor to a first bit line; coupling a gate of asecond vertical access transistor to the word line; coupling asource/drain region of the second vertical access transistor to theoutput of the other inverter; and coupling a second source/drain regionof the second vertical access transistor to a second bit line.
 14. Themethod of claim 13, further including coupling additional semiconductorrails to the access transistors, the additional semiconducting railsbeing parallel with the semiconductor rails of the inverters and beingdisposed on opposite sides of the semiconductor rails of the inverters.15. A method of forming a memory device, comprising: forming a number ofword lines and a number of bit lines that are disposed to form an array;forming a number of memory cells addressably disposed at intersectionsof the word lines and bit lines, wherein forming each memory cellincludes: forming a flip-flop that includes a cross-coupled pair ofinverters wherein forming each inverter includes: forming a pair ofcomplementary, vertical transistors, each transistor being formed fromone of a pair of single pillars of semiconducting material;interconnecting the gates of the inverter with a gate contact, the gatecontact acting as an input of the inverter; interconnecting the firstsource/drain regions of the complementary vertical transistors with ashunt, the shunt acting as an output of the inverter; coupling a gate ofa first vertical access transistor to a word line; coupling a firstsource/drain region of the first vertical access transistor to theoutput of one of the inverters; coupling a second source/drain region ofthe first vertical access transistor to a first bit line; coupling agate of a second vertical access transistor to the word line; coupling asource/drain region of the second vertical access transistor to theoutput of the other inverter; and coupling a second source/drain regionof the second vertical access transistor to a second bit line.
 16. Themethod of claim 15, further including coupling a word line decoder tothe word lines of the memory array that selectively activates the wordlines of the array.
 17. The method of claim 15, further includingcoupling a sense amplifier that is to the bit lines of the array ofmemory cells.
 18. The method of claim 17, further including coupling abit line decoder to the sense amplifier so as to select among the bitlines for reading and writing data to and from the memory cells.
 19. Themethod of claim 15, wherein coupling a second source/drain region of avertical access transistors to the bit lines includes coupling a secondsource/drain region of the vertical access transistors to a layer ofdoped semiconductor material that is integral with the secondsource/drain region of the vertical access transistors of the memorycells in the array.
 20. The method of claim 19, wherein coupling asecond source/drain region of a vertical access transistors to the bitlines includes coupling a second source/drain region of the verticalaccess transistors to bit lines formed below the body regions of thevertical access transistors.
 21. A method of forming a memory device,comprising: forming a number of word lines and a number of bit linesthat are disposed to form an array; forming a number of memory cellsaddressably disposed at intersections of the word lines and bit lines,wherein forming each memory cell includes: forming a flip-flop thatincludes a cross-coupled pair of inverters wherein forming each inverterincludes: forming a pair of complementary, vertical transistors, eachtransistor having a second source/drain region that is integral with abar of semiconductor material that interconnects the transistors withsimilar transistors in other cells in the array, each transistor beingformed from one of a pair of single pillars of semiconducting material;interconnecting the gates of the inverter with a gate contact, the gatecontact acting as an input of the inverter; interconnecting the firstsource/drain regions of the complementary vertical transistors with ashunt, the shunt acting as an output of the inverter; coupling a gate ofa first vertical access transistor to a word line; coupling a firstsource/drain region of the first vertical access transistor to theoutput of one of the inverters; coupling a second source/drain region ofthe first vertical access transistor to a first bit line; coupling agate of a second vertical access transistor to the word line; coupling asource/drain region of the second vertical access transistor to theoutput of the other inverter; and coupling a second source/drain regionof the second vertical access transistor to a second bit line.
 22. Amethod of forming a memory device, comprising: forming a number of wordlines and a number of bit lines that are disposed to form an array;forming a number of memory cells addressably disposed at intersectionsof the word lines and bit lines, wherein forming each memory cellincludes: forming a flip-flop that includes a cross-coupled pair ofinverters wherein forming each inverter includes: integrally forming apair of complementary, vertical transistors on a pair of complementarysemiconductor rails that run along a surface of asemiconductor-on-insulator wafer, each transistor being formed from oneof a pair of single pillars of semiconducting material; interconnectingthe gates of the inverter with a gate contact, the gate contact actingas an input of the inverter; interconnecting the first source/drainregions of the complementary vertical transistors with a shunt, theshunt acting as an output of the inverter; coupling a gate of a firstvertical access transistor to a word line; coupling a first source/drainregion of the first vertical access transistor to the output of one ofthe inverters; coupling a second source/drain region of the firstvertical access transistor to a first bit line; coupling a gate of asecond vertical access transistor to the word line; coupling asource/drain region of the second vertical access transistor to theoutput of the other inverter; and coupling a second source/drain regionof the second vertical access transistor to a second bit line.
 23. Themethod of claim 22, further including coupling additional semiconductorrails to the access transistors, the additional semiconducting railsbeing parallel with the semiconductor rails of the inverters and beingdisposed on opposite sides of the semiconductor rails of the inverters.24. A method of forming a memory cell, comprising: forming a flip-flopthat includes a cross-coupled pair of inverters wherein forming eachinverter includes: forming a pair of complementary, verticaltransistors, each transistor being formed from one of a pair of singlepillars of semiconducting material; interconnecting the gates of theinverter with a gate contact, the gate contact acting as an input of theinverter; interconnecting the first source/drain regions of thecomplementary vertical transistors with a shunt, the shunt acting as anoutput of the inverter; coupling a gate of a first vertical accesstransistor to a word line; coupling a first source/drain region of thefirst vertical access transistor to the output of one of the inverters;coupling a second source/drain region of the first vertical accesstransistor to a first bit line that is buried beneath the body of thefirst vertical access transistor; coupling a gate of a second verticalaccess transistor to the word line; coupling a source/drain region ofthe second vertical access transistor to the output of the otherinverter; and coupling a second source/drain region of the secondvertical access transistor to a second bit line that is buried beneaththe body of the second vertical access transistor.
 25. The method ofclaim 24, wherein coupling a second source/drain region of a verticalaccess transistors to the bit lines includes coupling a secondsource/drain region of the vertical access transistors to a layer ofdoped semiconductor material that is integral with the secondsource/drain region of the vertical access transistors.
 26. The methodof claim 24, wherein interconnecting the gates of the inverter with agate contact includes interconnecting the gates of the inverter with atungsten gate contact that is disposed along a side of thecomplementary, vertical transistors of each inverter.
 27. The method ofclaim 9, wherein forming a memory cell includes forming a memory cellthat uses a surface area of a semiconductor wafer that is approximately32 feature (F) squares.
 28. The method of claim 24, wherein coupling asecond source/drain region of a vertical access transistors to the bitlines includes coupling a second source/drain region of the verticalaccess transistors to an integral bar of semiconductor material thatinterconnects the transistors with similar transistors in other cells.29. A method of forming a memory cell, comprising: forming a flip-flopthat includes a cross-coupled pair of inverters wherein forming eachinverter includes: forming a pair of complementary, verticaltransistors, each transistor being formed from one of a pair of singlepillars of semiconducting material; interconnecting the gates of theinverter with a gate contact, the gate contact acting as an input of theinverter; interconnecting the first source/drain regions of thecomplementary vertical transistors with a tungsten shunt, the tungstenshunt acting as an output of the inverter; coupling a gate of a firstvertical access transistor to a word line; coupling a first source/drainregion of the first vertical access transistor to the output of one ofthe inverters; coupling a second source/drain region of the firstvertical access transistor to a first bit line that is buried beneaththe body of the first vertical access transistor; coupling a gate of asecond vertical access transistor to the word line; coupling asource/drain region of the second vertical access transistor to theoutput of the other inverter; and coupling a second source/drain regionof the second vertical access transistor to a second bit line that isburied beneath the body of the second vertical access transistor. 30.The method of claim 29, wherein interconnecting the first source/drainregions of the complementary vertical transistors with a tungsten shuntincludes coupling the tungsten shunt to a top surface of the firstsource/drain region of one of the vertical, access transistors.
 31. Amethod of forming a memory cell, comprising: forming a flip-flop thatincludes a cross-coupled pair of inverters wherein forming each inverterincludes: integrally forming a pair of complementary, verticaltransistors on a pair of complementary semiconductor rails that runalong a surface of a semiconductor-on-insulator wafer, each transistorbeing formed from one of a pair of single pillars of semiconductingmaterial; interconnecting the gates of the inverter with a gate contact,the gate contact acting as an input of the inverter; interconnecting thefirst source/drain regions of the complementary vertical transistorswith a shunt, the shunt acting as an output of the inverter; coupling agate of a first vertical access transistor to a word line; coupling afirst source/drain region of the first vertical access transistor to theoutput of one of the inverters; coupling a second source/drain region ofthe first vertical access transistor to a first bit line that is buriedbeneath the body of the first vertical access transistor; coupling agate of a second vertical access transistor to the word line; coupling asource/drain region of the second vertical access transistor to theoutput of the other inverter; and coupling a second source/drain regionof the second vertical access transistor to a second bit line that isburied beneath the body of the second vertical access transistor. 32.The method of claim 13, further including coupling additionalsemiconductor rails to the access transistors, the additionalsemiconducting rails being parallel with the semiconductor rails of theinverters and being disposed on opposite sides of the semiconductorrails of the inverters.
 33. A method of forming a memory device,comprising: forming a number of word lines and a number of bit linesthat are disposed to form an array; forming a number of memory cellsaddressably disposed at intersections of the word lines and bit lines,wherein forming each memory cell includes: forming a flip-flop thatincludes a cross-coupled pair of inverters wherein forming each inverterincludes: forming a pair of complementary, vertical transistors, eachtransistor being formed from one of a pair of single pillars ofsemiconducting material; interconnecting the gates of the inverter witha gate contact, the gate contact acting as an input of the inverter;interconnecting the first source/drain regions of the complementaryvertical transistors with a shunt, the shunt acting as an output of theinverter; coupling a gate of a first vertical access transistor to aword line; coupling a first source/drain region of the first verticalaccess transistor to the output of one of the inverters; coupling asecond source/drain region of the first vertical access transistor to afirst bit line that is buried beneath the body of the first verticalaccess transistor; coupling a gate of a second vertical accesstransistor to the word line; coupling a source/drain region of thesecond vertical access transistor to the output of the other inverter;and coupling a second source/drain region of the second vertical accesstransistor to a second bit line that is buried beneath the body of thesecond vertical access transistor.
 34. The method of claim 33, furtherincluding coupling a word line decoder to the word lines of the memoryarray that selectively activates the word lines of the array.
 35. Themethod of claim 33, further including coupling a sense amplifier that isto the bit lines of the array of memory cells.
 36. The method of claim35, further including coupling a bit line decoder to the sense amplifierso as to select among the bit lines for reading and writing data to andfrom the memory cells.
 37. The method of claim 33, wherein coupling asecond source/drain region of a vertical access transistors to the bitlines includes coupling a second source/drain region of the verticalaccess transistors to a layer of doped semiconductor material that isintegral with the second source/drain region of the vertical accesstransistors of the memory cells in the array.
 38. A method of forming amemory device, comprising: forming a number of word lines and a numberof bit lines that are disposed to form an array; forming a number ofmemory cells addressably disposed at intersections of the word lines andbit lines, wherein forming each memory cell includes: forming aflip-flop that includes a cross-coupled pair of inverters whereinforming each inverter includes: forming a pair of complementary,vertical transistors, each transistor having a second source/drainregion that is integral with a bar of semiconductor material thatinterconnects the transistors with similar transistors in other cells inthe array, each transistor being formed from one of a pair of singlepillars of semiconducting material; interconnecting the gates of theinverter with a gate contact, the gate contact acting as an input of theinverter; interconnecting the first source/drain regions of thecomplementary vertical transistors with a shunt, the shunt acting as anoutput of the inverter; coupling a gate of a first vertical accesstransistor to a word line; coupling a first source/drain region of thefirst vertical access transistor to the output of one of the inverters;coupling a second source/drain region of the first vertical accesstransistor to a first bit line that is buried beneath the body of thefirst vertical access transistor; coupling a gate of a second verticalaccess transistor to the word line; coupling a source/drain region ofthe second vertical access transistor to the output of the otherinverter; and coupling a second source/drain region of the secondvertical access transistor to a second bit line that is buried beneaththe body of the second vertical access transistor.
 39. A method offorming a memory device, comprising: forming a number of word lines anda number of bit lines that are disposed to form an array; forming anumber of memory cells addressably disposed at intersections of the wordlines and bit lines, wherein forming each memory cell includes: forminga flip-flop that includes a cross-coupled pair of inverters whereinforming each inverter includes: integrally forming a pair ofcomplementary, vertical transistors on a pair of complementarysemiconductor rails that run along a surface of asemiconductor-on-insulator wafer, each transistor being formed from oneof a pair of single pillars of semiconducting material; interconnectingthe gates of the inverter with a gate contact, the gate contact actingas an input of the inverter; interconnecting the first source/drainregions of the complementary vertical transistors with a shunt, theshunt acting as an output of the inverter; coupling a gate of a firstvertical access transistor to a word line; coupling a first source/drainregion of the first vertical access transistor to the output of one ofthe inverters; coupling a second source/drain region of the firstvertical access transistor to a first bit line that is buried beneaththe body of the first vertical access transistor; coupling a gate of asecond vertical access transistor to the word line; coupling asource/drain region of the second vertical access transistor to theoutput of the other inverter; and coupling a second source/drain regionof the second vertical access transistor to a second bit line that isburied beneath the body of the second vertical access transistor. 40.The method of claim 39, further including coupling additionalsemiconductor rails to the access transistors, the additionalsemiconducting rails being parallel with the semiconductor rails of theinverters and being disposed on opposite sides of the semiconductorrails of the inverters.